Electroplating



Patented Sept. 5, 1939 ELECTROPLATING Harold J. Barrett and Christian J. Wernlund, Ni-

agara Falls, N. Y., assignors to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation oi Delaware No Drawing. Application July 13, 1936, Serial No. 90,382

- 12 Claims.

This invention relatesto the art of electroplating metals and more particularly to a novel type of addition agent for electroplating baths to improve the qualities of the electrodeposits.

The electrodeposition of a metal from solutions of metallic compounds often results in an electrodeposit which may be rough, spongy or dark colored or otherwise is unsuited for the purpose for which it is intended. Heretoi'ore various attempts have been made to improve electrodeposits by adding to the electroplating solution various so-called addition agents which advantageously affect the character of the electrodeposit. Such addition agents maymake electrodeposits more dense and inhibit the natural tendency of the metal to form trees" or deposit in a spongy or porous condition. A further effect sometimes gained by the use of such addition agents is to improve the appearance of the electrodeposit, for example, to inhibit the formation of dark colors or to obtain a certain degree of brightness in the plated product. A. great variety of chemical compounds, both organic and inorganic, have been proposed as addition agents for improving electrodeposits of various metals e. g., tin, copper, zinc, lead, cadmium, and alloys such as brass and bronze and others. Among such addition agents may be mentioned, essential oils such as oil of cloves, citronella, oil of cedar, peppermint, sandalwood; terpene-like materials such as thymol and menthol; natural gums, e. g., licorice and camphor; carbohydrate materials such as starch, dextrln and various sugars, e. g., dextrose; alkaloids such as quinine; protein materials such as peptone and gelatin; phenols e. g., hydroquinone, resorcinol or pyrogallol and various other organic substances such as nitrobenzene, aniline, glycerin and others. Some of these have been proposed for use in a variety of difierent electroplating operations, for example, glue or gelatin which has been proposed for electroplating various me als. The majority of these, however, are not universal in their effect; that is many have been proposed each only for the electrodeposition of a single metal.

A number of metals, e. g., zinc and cadmium, may readily be plated in the form of dense coatings which are satisfactory as protection against corrosion but which tend to be somewhat rough and do not have a high degree of brightness when plated in the absence of addition agents by the ordinary known means. For many purposes it is desirable to produce rust resisting coatings which not only are dense and'non-porous but also have a considerable degree of brightness and an attractive appearance. Recently, means have been discovered for electroplating zinc in a bright form, for example, the method described and claimed in the pending patent applications by Floyd F. Oplinger, Serial No. 752,704, filed November 12, 1934, and Serial No. 46,163, filed October 22, 1935, which comprises first purifying a zinc cyanide plating bath to remove traces of heavy metal impurities such as lead and cadmium and finally treating the electrodeposit with a dilute acidic oxidizing solution such as dilute nitric acid. In order to successfully produce bright zinc eleetrodeposits by this method it usually is necessary to take care that the current density is not too low. Difliculty is sometimes experienced in producing satisfactory bright deposits by the Oplinger method when it is used in barrel plating operations, since in barrel plating certain of the work being plated often becomes exposed to rather low current densities. Consequently, in barrel plating special precautions must be taken in order to make sure that all pieces are exposed to a sufiiciently high current density in order to obtain the bright zinc deposit. In such operations where a bright deposit is desired, an addition agent which will cause the production of a brighter electrodeposit is of especial value.

An object of the present invention is to provide an improved method for electroplating metals including zinc, nickel, cadmium, tin, and copper and their alloys by means which results in improved electrodeposited coatings. A further object is to provide a novel addition agent for electroplating baths which will cause the formation of improved electrodeposits having superior hardness, 'denseness, brightness, and other advantageous properties. A still further objectis to provide an improved method for electrodepositing bright zinc and bright nickel coatings on steel and other metals. Other objects will be hereinafter apparent.

The above objects are obtained in accordance with our invention by adding to the electroplating bath polyvinyl alcohol or a chemical derivative of polyvinyl alcohol which is more or less soluble in water. Polyvinyl alcohol is a resinouslike material which may be obtained by hydrolysis of polyvinyl acetate, which in turn is made by polymerizing vinyl acetate. Polyvinyl alcohol may be made in a number of grades or modifications which differ one from another by their, degree of polymerization. All of these modifications are fairly soluble in water, for example, 10% by weight aqueous solution of these polyvinyl alcohols may readily be made although the actual solubility will vary somewhat depending upon the degree of polymerization. Various derivatives of polyvinyl alcohol may be made by different chemical reactions, which derivatives may be either water soluble or capable of being swelled by contact with water. We use the term hydrophilic polyvinyl alcohol derivatives hereinafter and in the appended claims to designate tit such materials made from polyvinyl alcohol which are either water soluble or at least are capable of being swelled by the action of water. Also, the term hydrophllic polyvinyl alcohol compound" is used herein and in the appended claims to designate generically polyvinyl alcohol and hydrophilic polyvinyl alcohol derivatives. Such hydrophilic materials either form true solutions in water or are capable of being converted into colloidal suspensions (hydrosols) in water. One example of such hydrophilic polyvinyl alcohol derivatives is the so-called partial ester of polyvinyl alcohol which may be made by only partially hydrolyzing a polyvinyl ester. If the hydrolysis of the ester is carried out to a sufiicient extent, the resulting compound, which contains both ester groups and free hydroxy groups in the same molecule, will be hydrophilic in nature. Other similar derivatives of polyvinyl alcohol are the partial polyvinyl alcohol acetals which may be made by incompletely condensing polyvinyl alcohol with aldehydes such as formaldehyde or acetaldehyde and the partial ethers of polyvinyl alcohol which may be made by partially etherifying polyvinyl alcohol with some other alcohol such as glycerin, methanol, or the like. Also certain esters of polyvinyl alcohol may be made which are sufiiciently water soluble for the purposes of the present invention, for ex.- ample, the boric acid ester and the hydrosulfuric acid ester; the latter ester may be made by reacting polyvinyl alcohol with sulfuric acid.

We have discovered that the addition of unexpectedly small amounts of polyvinyl alcohol to an electroplating bath has a marked and advantageous efiect on the character of the resulting metal electrodeposit. The polyvinyl alcohol not only causes a harder and denser electrodeposit to be formed but also in many cases has a very marked effect as a brightening agent especially in the electroplating of zinc, cadmium and nickel. It is especially well adapted for the production of bright zinc deposits, for example,

by adding polyvinyl alcohol or its derivatives to a zinc cyanide electroplating bath and practicing the above mentioned Oplinger method for producing bright zinc. The addition of the polyvinyl alcohol in proper proportions enables the electrodeposition of bright zinc electrodeposits of high quality, often without the necessity of subsequent bright dipping, at unusually low current densities. Thus, by practice of the present invention one is able to directly electroplate satisfactorily bright deposits on small articles by means of the barrel plating operation. Furthermore, polyvinyl alcohol appears to increase the covering power of the electroplating bath in barrel plating especially in the electrodeposition of zinc, which is of value when plating recessed articles.

We have discovered that the addition of polyvinyl alcohol or its derivatives to a nickel plating bath results in smooth, bright, lustrous, nickel deposits which are equal in appearance and smoothness to polished nickel. In using polyvinyl alcohol as addition agent for plating nickel it is advisable to use caution not to add too much. Because of the hardening action of the polyvinyl alcohol, much in excess over that required to impart the desired brightness to the electrodeposit often makes the electroplated nickel so brittle that it readily chips off from the base metal. However, by avoiding such excess, we have been able to directly plate bright, mirrorlike nickel deposits of high qualitY. Wh le the Optimum amount of polyvinyl alcohol will vary widely, depending on the nickel bath used and plating conditions, we prefer. to add from 0.005 to 0.05 gram per gallon for nickel plating. Generally somewhat larger amounts of the polyvinyl alcohol derivatives will be required. For example, we have obtained excellent results by employing around 0.025 gram per gallon of polyvinyl alcohol in a nickel sulfate plating bath.

The tendency for the deposit to become brittle also can be inhibited by avoiding high cathode current densities; we prefer to operate at a current density not greater than about 25 amperes per square foot, e. g. at 10 to 20 amperes per square foot- We prefer to operate the nickel plating bath at a moderately warm temperature, e. g. 35 to 40 C. The various known nickel plating baths are suitable .for practicing our invention.

The amount of polyvinyl alcohol or hydrophilic polyvinyl alcohol derivative to be added to the electroplating bath in general will vary, depending upon the metal to be plated, the-degree of brightness or hardness required and the solubility of the polyvinyl compound selected. In general, we may add from 0.01 to 0.5 gram per liter of polyvinyl alcohol; in most cases from 0.02 to 0.1 gram per liter will be found most suitable. Since the addition of polyvinyl alcohol to the electroplating bath in general causes electrodeposits to become harder, large amounts of this addition agent in general should be avoided; if too large an amount, e. g. more than about 1 gram per liter, is used, the electrodepos'it may become too hard and brittle and have a tendency to flake oil when the plated article is flexed or hammered. This hardening efiect of course varies with different metals so that in some cases the precaution against adding too much of the polyvinyl alcohol is more important than in others. For example, in electroplating nickel in accordance with our invention it is especially essential that care be used not to introduce too much polyvinyl alcohol into the plating bath in order to prevent the formation of brittle deposits. However, by properly proportioning the amount of polyvinyl alcohol, for example, as disclosed in an example given below, we have been able to produce brilliant deposits of nickel which had satisfactory adherence and ductility.

As stated above, the present invention is especially well adapted to the electrodeposition of bright zinc. Although we have been able to produce superior zinc deposits having an unusual degree of brightness from various types of zinc plating baths, for example the ordinary zinc cyanide bath or common zinc sulphate bath, by the addition of polyvinyl alcohol thereto. We prefer to employ the type of bath and method of electroplating described in the above mentioned patent applications by Floyd F. Oplinger. In one method for carrying out this method in accordance with the present invention, we first make a plating bath containing zinc cyanide, sodium cyanide, and an alkalining agent such as sodiumhydroxide, preferably containing not less than about 6 oz./gal. of zinc cyanide.

We then purify the bath to remove any traces of heavy erably the bath then is allowed to stand to settle out the heavy metal precipitate or is filtered. Before electroplating with the purified bath,-we add thereto polyvinyl alcohol in the form of aqueous solution in an amount equivalent to 0.02

to 0.1 gram per liter of polyvinyl alcohol. Alternatively, we may add a hydrophilic polyvinyl dequired no further treatment is necessary. However, a slight improvement in the appearance of the electroplated article usually may be obtained by bright dipping in nitric acid or other suitable bright dipping solution.

Our invention is further illustrated by the following examples:

EXAMPLE I Plating bath.

Zinc cyanide '75 g./l. Sodium cyanide 37.5 g./l. Caustic soda '75 g./l. Sodium sulfide 0.25 g./l. Polyvinyl alcohol 0.026, 0.078, 0.13, 0.26, and

(speratzng conditions Anodes Electrolytic zinc- Hg. Ratio anode to cathode surface 3 to 1. Temperature 25 to 30 C. Hot rolled sheet steel Time of plating 15 minutes.

Plating data As little as 0.026 g./l. (0.1 g./gai.) of polyvinyl alcohol, introduced as a 5% aqueous solution.

produced good, bright-dippable, brown filmed deposits at 20 to 50 amps/sq. it. The addition of 0.078 g./l. (0.3 g./gal.) of polyvinyl alcohol produced semi-bright, yellowish filmed deposits at 10 amps/sq. ft. which yielded an excellent bright zinc surface on bright dipping. The 'deposits at 20, 30, 40 and 50 amps/sq. ft. duplicated the above results. The use of 0.13 g.'/l. v(0.5 g./gal.) of polyvinyl alcohol produced excellent bright-dippable plate, which have a brownish sheen at 10, 20, 30, 40 and 50 amps/sq. ft. This is the optimum concentration of brightener in this bath for still plating for most purposes. The use of 0.26 g./l. (1 g./gal.) of polyvinyl alcohol duplicated the above results at 10, 30 and 50 amps/sq. it. except that at 30 and 50 amps/sq. ft. the edges of the plate were slightly dull. The use of 0.52 g./l. (2 g./ga1.) of polyvinyl alcohol produced bright-dippable deposits only at 30 amps/sq. ft.

The chief beneficial efiect of polyvinyl alcohol in the still bright zinc solution is that bright plate is produced at much lower current densities than is practical without its use, thus enabling one to produce bright plate in deeply recessed articles. It was also observed that the use of the addition agent enabled the production of'brighter deposits than those normally produced at 30 amps/sq. ft. without the'addition agent. This is useful in that a more pleasing finish can be produced on hot rolled and other rough surfaced articles.

. Exmui II The plating baths used were as follows:

Solution No.1 No.2 No.3

.1. .1. .z. Zinc cyanide 3 3 3 Sodium cyanide. 3.07 18.0 33 Causticaoda 52.6 52.6 52.6 Sodium suiilde.. 0.25 0.25 0.25 Polyvinyiaicohoi 0.13 0.13 0.20

Operating conditions Apparatus 1.0 gallon commercial oblique barrel (rubber lined). Anodes Electrolytic zinc in circular anode basket. Temperature 22 to 43 C. Current--. 50 to 75 amps/40 to 50 lb.

load at '7 volts. Materials placed- Bolts, nuts and nails.

Time of plating 1 to 4 hours.

Bath No. 1.-Produced brownish filmed bright deposits which bright dipped to yield bright deposits.

Bath No. 2.-Heavy (2 hour) deposits were very bright and only slightly brownish, the brownish. color being removed on bright dipping.

Bath No. 3.'li'hese deposits had a good bright appearance without bright dipping. Bright dipping increased the brightness.

The maximum brightness of plate in barrel platingwas obtained with 0.13 g./l. (0.5 g./gal.)- of polyvinyl alcohol. The amount of addition agent required. will depend somewhat upon the nature of the work being plated and the current density used. In general the optimum amount of brightener required for barrel plating from this. bath may vary from about 0.026 g./l. to 0.13 g./l. (0.1 gJgal. to 0.5 g./gal.) and may be as high as 0.26 g./l. (l g./gal.) for producing maximum brightness on rough surfaces.

EXAMPLE III The following zinc platingbath was used to plate steel, using as addition agents three different grades of polyvinyl. alcohol, having difierent molecular weights:

Zinc cyanide '15 g./l. Sodium cyanide"- 37.5 g./l. Caustic soda 75 g./l.

Sodium sulfide 0.25 g./l. Pdlyvinyl'aLl'cohoL- 0.052, 0.13, 0.26 and 0.52 g./l.

Operating conditions The results obtained may be summarized as follows: At low current densities. l0 amps./sq. it., all .three grades of polyvinyl alcohol at concentrations of 0.052'g./l. (0.2 g./gal.) produced excellent bright plate, while deposits from the above solution with no addition agent were grayish white atthe' above current density. With concentrations of 0.052 to 0.26 g./l. (0.2 to 1.0 g./gal.)

all three polyvinyl compounds produced excellent bright plate. At 0.52 g./l. (2 g./gal.) all three compounds produced dull deposits which did not bright dip except at 30 amps/sq. ft. At low addition agent concentration, the low molecular weight compound produced slightly brighter deposits; at higher concentrations the high molecular weight compound produced a slightly brighter plate. on the whole, however, all three of these polyvinyl alcohols proved to be excellent brightening agents.

EXAMPLE IV A cadmium plating solution was made up containing the equivalent of 30 g./l. of cadmium oxide (CdO) and 75 g./l. of sodium cyanide. The solution was divided into two portions and to one portion was added 0.025 g./l. of polyvinyl alcohol dissolved in water. Electrodeposits then were made from each portion, using pure cadmium anodes and electroplating in each case at 30 amps./sq. ft. cathode current density, for 10 minutes. The electrodeposit obtained from the solution containing no polyvinyl alcohol was the characteristic dull white cadmium plate. That obtained from the bath containing the polyvinyl alcohol had a bright and shiny appearance, similar to bright-dipped cadmium.

Examrra V Still plating baths used (ingredients in g./l.):

Time of plating-.- 30, 15 and 10 minutes. Cathode current densities 10, 30, 60 and amps./sq. ft.

Plating data.

The straight sulfate bath (No. 1) produced the usual smooth, dull, grayishwhite sulfate coatings, containing numerous small pits at 60 and 100 amps/sq. it.

In contrast to the above, bath No. 2 produced characteristic addition agent coatings of two kinds. At 60 and at 100 amps./sq. ft. the deposits were smooth, semi-bright and ductile. These yielded bright surfaces when dipped in a nitric acid solution which closely resemble the new cyanide bright dipped zinc coating.

Bath No. 3 operated at 100 amps/sq. ft. yielded deposits which were good smooth semi-bright deposits, which bright dipped readily in nitric acid solution.

Sodium sulfide 1.0 g./l. sulfated polyvinyl alcohol 0.025, 0.05 and 0.125 g./l.

0.07, 0.19, 0.47 g./ga1. .The sulfated polyvinyl alcohol was prepared by reacting 5 parts by weight of polyvinyl alcohol with 3.5 parts of chlorosulfonic acid in 100 parts of chloroform maintained at a temperature of about 3 C. Finely divided polyvinyl alcohol was suspended in the chloroform and the chlorosulfonic acid was added slowly, while the chloroform suspension was stirred vigorously. Hydrogen chloride was evolved and the reaction mixture became orange in color as the reaction proceeded. After continuingagitation for 20 minutes, the mixture was neutralized with 10% sodium hydroxide solution and the resulting flocculent material was filtered off and dried over night. The resulting product was a pinkish, rubbery-like material, soluble in water, although less soluble than polyvinyl alcohol. A'1% aqueous solution of the sulfated material was used to add to the zinc plating bath.

Operating conditions Anodes Electrolytic zinc-%.% Hg. Ratio anode to cathode surface 3.to 1. Temperature 25 to 30 0. Cold rolled sheet steel Time of plating 20 to 60 min.

Plating data Comparative data of the sulfated alcohol and polyvinyl alcohol as brightening agents are given below:

The most striking results were obtained with both products at current densities of 10 to 30 amps/sq. ft. As little as 0.025 g./l. (0.07 g./gal.) of the sulfated polyvinyl alcohol produced a marked brightening effect at 20 amps./sq. ft. although the brightening efiect was not as great as that produced by the same amount of polyvinyl alcohol. The smoothest deposits were obtained by using 0.05 g./l. (0.19 g./gal.) of sulfated polyvinyl alcohol. In general, 3 to 5 times as much of the sulfated alcohol as of polyvinyl alcohol is necessary to produce the same cathode brightening effect.

Emu VII Plating bath Zinc cyanide 75 g./l. Sodium cyanide 52.5 g./l. Caustic soda 75 g./l. Sodium sulphide 1.0 g./l. Polyvinyl alcohol boric acid ester 0.025 and 0.05 g./1.

0.07, 0.19 g./gal.

The boric acid ester was prepared by adding 10 cc. of a 1% aqueous borax solution to cc. of a 2% aqueous solution of polyvinyl alcohol. After stirring for about 5 minutes, a somewhat gelatinous solution of the ester was formed. This-solution was used as the addition agent.

Operating conditions Anodes Electrolytic zinc-- A% Ratio anode to cathode surface 3 to 1. Temperature 25 to 30 C. Cold rolled sheet steel Time of plating 10 to 20 min.

Plating data Comparative data of the polyvinyl alcohol boric acid ester and polyvinyl alcohol as brightening agents: Concentrations of the boric acid ester of polyvinyl alcohol up to 0.05 g./l., gave identical results to that obtained by using the same concentration of polyvinyl alcohol as described in Example I.

It is to be understood that our herein described invention is not restricted to the exact modes of operation given herein by way of example. Likewise the invention is not restricted to the use of the particular polyvinyl compounds mentioned, but includes the addition to the plating bath of various hydrophllic polyvinyl compounds and likewise compounds which may become changed in the bath to polyvinyl alcohol or hydrophilic polyvinyl derivative. Thus, for example, esters of polyvinyl alcohol such as polyvinyl acetate or polyvinyl formate, which may become hydrolyzed in the plating bath to form in situ hydrophilic compounds such as polyvinyl alcohol or partial esters of polyvinyl alcohol, may be used without departing from the spirit and scope of our invention.

We claim:

1. The process which comprises electroplating zinc from a solution containing in solution an amount of polyvinyl alcohol eilective to produce a bright zinc deposit, not greater than 1 gram per liter.

2. The process which comprises electroplating zinc from a solution containing 0.01 to 0.5 gram per liter of polyvinyl alcohol.

3. A zinc plating bath containing in solution an amount of polyvinyl alcohol effective to produce a bright zinc deposit, not greater than 1 gram per liter.

4. A zinc plating bath containing 0.01 to 0.5 gram per liter of polyvinyl alcohol.

5. The process which comprises electroplating zinc from a solution containing a hydrophilic' polyvinyl alcohol compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters and polyvinyl acetals in an amount effective to produce a bright zinc deposit, not greater than 1 gram per liter.

6. The process which comprises electroplating zinc from a cyanide solution containing a hydrophilic polyvinyl alcohol compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters and polyvinyl acetals, in an amount effective to produce a. bright zinc deposit, not greater than 1 gram per liter.

'7. The process which comprises adding suflicient soluble sulfide to a zinc cyanide solution to substantially completely precipitate heavy metal impurities therefrom, dissolving in said solution a hydrophylic polyvinyl compound selected from the group comprising polyvinyl alcohol, polyvinyl esters and polyvinyl acetals in an amount effective to produce a bright zinc deposit, not greater than 1 gram per liter and thereafter plating zinc from said solution.

8. The process which comprises electroplating zinc from a cyanide solution containing in solution an amount of polyvinyl alcohol effective to produce a bright zinc deposit, not greater than 1 gram per liter.

9. The process which comprises adding sufficient soluble sulfide to a zinc cyanide solution to substantially completely precipitate heavy metal impurities therefrom, dissolving in said solution an amount of polyvinyl alcohol efiective to produce a bright zinc deposit, not greater than 1 gram per liter and thereafter plating zinc from said solution.

10. A zinc electroplating bath which contains a. hydrophllic polyvinyl compound selected from the. group consisting of polyvinyl alcohol, polyvinyl esters and polyvinyl alcohol acetals in an amount effective to produce abright zinc deposit, not greater than 1 gram per liter.

11. A zinc cyanide electroplating bath which contains a hydrophilic polyvinyl compound selected from the group consisting of polyvinyl alcohol, polyvinyl esters and polyvinyl alcohol acetals in an amount effective to produce a. bright zinc deposit, not greater than 1 gram per liter.

12. A zinc electroplating bath which contains zinc cyanide and in solution an amount of polyvinyl alcohol eflfective to produce a bright zinc deposit, not greater than 1 gram per liter.

HAROLD J. BARRETT. CHRISTIAN J. WERNLUND. 

